Naftz et al. Geochemical Transactions 2011, 12:4 http://www.geochemicaltransactions.com/content/12/1/4 RESEARCHARTICLE Open Access A 50-year record of NOx and SO2 sources in precipitation in the Northern Rocky Mountains, USA David L Naftz1*, Paul F Schuster2, Craig A Johnson3 Abstract Ice-core samples from Upper Fremont Glacier (UFG), Wyoming, were used as proxy records for the chemical composition of atmospheric deposition. Results of analysis of the ice-core samples for stable isotopes of nitrogen δ15 − δ34 2− − 2− ( N, NO3 ) and sulfur ( S, SO4 ), as well as NO3 and SO4 deposition rates from the late-1940s thru the early-1990s, were used to enhance and extend existing National Atmospheric Deposition Program/National Trends Network (NADP/NTN) data in western Wyoming. The most enriched δ34S value in the UFG ice-core samples coincided with snow deposited during the 1980 eruption of Mt. St. Helens, Washington. The remaining δ34S values were similar to the isotopic composition of coal from southern Wyoming. The δ15N values in ice-core samples representing a similar period of snow deposition were negative, ranging from -5.9 to -3.2 ‰ and all fall within the δ15N values expected from vehicle emissions. Ice-core nitrate and sulfate deposition data reflect the sharply increasing U.S. emissions data from 1950 to the mid-1970s. Introduction expressed as δ34S, were monitored in bulk snowpack The chemical quality of snowfall deposited in high-eleva- samples collected from a network of 52 high-elevation tion areas in the Rocky Mountain region can be affected sites in the Rocky Mountains from 1993 to 1999 [5]. by energy generation and associated population growth The δ34S values indicated that snowpack S in high-ele- [1,2]. High elevation areas in the Wind River Range vation areas is primarily derived from anthropogenic (WRR) of Wyoming (figure 1) exceed 4 km above sea sources [5]. level and are adjacent to areas of accelerating energy Changes in δ34S values in firn and ice-core samples development [3]. For example, over 3,000 natural gas have been used to reconstruct changes in sulfate sources wells are being installed in the Green River Basin, directly to central Asia, Greenland, and Antarctica. The variation west of the WRR. Full development of the Jonah gas field in δ34S values in a firn core from central Asia allowed could result in the production of 1,480 metric tons/yr of for the identification of S derived from marine evapor- NOx and 25.7 metric tons/yr of SO2 [3]. ites (+15 ‰) during high dust deposition events and Thin soils and dilute surface-water systems in high- anthropogenic emissions (+5.4 ‰) [6]. Preindustrial δ34S elevation areas have limited capacities to buffer signatures in Greenland ice cores were comprised of increased acidity associated with the airborne contami- marine biogenic emissions, continental dust sources, nants of NOx and SO2. Trends in precipitation chemis- background volcanism, and continental biota [7]. Begin- try at NADP/NTN sites in the western United States ning in 1870 A.D., δ34S signatures in the ice-core sam- have indicated an increase in total N deposition and a ples indicated anthropogenic S sources. − decrease in SO2 deposition from 1981-1998 [4]. In In Antarctica, shallow firn cores collected from the 4 δ34 addition to monitoring trends in N and S deposition, South Pole contained S values that were used to con- the isotopic composition of snow, firn, and ice has been firm and contrast the different S isotopic signals between used to differentiate natural and anthropogenic solute the low-latitude Agunge volcanic eruption in 1963 and − background marine biogenic sulfate [8]. Composited ice- sources. Stable isotope ratios of sulfur in SO2 , 4 core samples representing coastal and plateau regions on the Antarctic ice sheet were found to have similar δ34S * Correspondence: [email protected] 1 values over the past 1,100 years, indicative of no temporal U.S. Geological Survey, 2329 W. Orton Circle, Salt Lake City, UT 84119, USA δ34 Full list of author information is available at the end of the article change in influencing sources of S [9]. Values of Sin © 2011 Naftz et al; licensee Chemistry Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited Naftz et al. Geochemical Transactions 2011, 12:4 Page 2 of 10 http://www.geochemicaltransactions.com/content/12/1/4 109O37’30” 109O36’15” 43O08’15” WY98 Upper Fremont Glacier Wind River Range WY06 DH-98-4 DH-91-1 43O07’30” Base from U.S. Geological Survey Fremont Peak North, 1:24,000 1993 0 0.2 KM Wind CONTOUR INTERVAL 12.2 METERS River NATIONAL GEODETIC VERTICAL DATUM OF 1929 Range Wyoming Explanation DH-91-1 Ice-coring site WY06 National Atmospheric Deposition Program (NADP) Monitoring site Figure 1 Location of NADP/NTN and ice-coring sites in Wind River Range, Wyoming. two ice cores from east Antarctica over the complete gla- information is needed to confirm this trend and differ- cial/interglacial cycle were significantly lighter than pre- entiatesourceareavs.post-depositionalprocessesdur- vious measurements of δ34S from the South Pole [10]. A ing the firn-to-ice transition [15]. A more recent ice- likely mechanism for the observed isotopic difference was core record from Greenland, spanning deposition from Rayleigh-type fractionation as S species are oxidized and 1718 to 2006, revealed a clear trend of decreasing δ15N − ‰ ‰ transported toward the East Antarctic Plateau [10]. Mea- ( NO3 )valuesfrom11 (pre-industrial) to -1 surements of δ34S values in an ice core collected from (~1996-2006) [16]. west Antarctica were found to be a mixture of marine Although previous studies have shown that δ15N and volcanic S sources during the time period from − (NO3 ) values of snow and ice samples have excellent 1935-1976. [11]. − potential for providing information on NO sources, Stable isotopes have also been used to gain a better 3 post-depositional changes in the concentration and iso- understanding of N sources in atmospheric deposition. − topic composition of NO needs to be considered Historical records of the isotopic composition of N2Oin 3 δ15 − trapped gases (ice cores) from Greenland and interstitial [17,18]. Year-round measurement of N(NO3 ) values air (snowpack) from the South Pole have been used to in snow pits from Dome C, Antarctica, indicated strong − differentiate between natural and anthropogenic sources NO 15 enrichment relative to atmospheric 3 and loss of [12]. The δ N value of atmospheric N Ohasdropped − 2 NO mass from the snow surface due to UV-photolysis by 1.7 ‰ during the 20th century, likely due to increas- 3 ing agricultural activities [12]. In support of these mea- [17]. In contrast to the Antarctica results, a similar study in Greenland [18] indicated minimal influence of surements, simulations of N2O(g) have indicated a -1.8 − 15 δ15 ‰ shift in δ N over the last two centuries, primarily photolysis on the isotopic composition of N(NO3 ) δ15 − due to anthropogenic influences [13]. The NofNO3 in firn and ice. One possible reason for the different in wet deposition from 33 sites in the northeastern Uni- research results between the two studies may be the ted States was strongly correlated with the location of higher snow accumulation rates at the Greenland study coal-fired powerplants [14]. Ice-core samples from site [18]. Higher rates of snow accumulation were found δ15 − Greenland indicated decreasing N(NO3 )values to mitigate the magnitude of post-depositional proces- − − with increasing NO3 concentrations; however, more sing and loss of NO3 in the snowpack [19]. Naftz et al. Geochemical Transactions 2011, 12:4 Page 3 of 10 http://www.geochemicaltransactions.com/content/12/1/4 Glaciers present in the high elevation regions of the stored in snow vaults until removal from the site to a WRR, Wyoming, present a unique opportunity to couple freezer truck via a 10-minute helicopter flight. The UFG short-term (1980 to present) NADP/NTN data (sites ice cores are currently archived at the National Ice Core WY06 and WY98) trends in the chemistry of atmo- Laboratory (NICL) in Lakewood, Colorado. spheric deposition with similar and longer term data preserved in glacial ice from the nearby UFG (figure 1). Laboratory The UFG is the only glacier within the continental U.S. Ice-core samples were melted according to strict protocols where ice cores have been documented to contain [21] to minimize sample contamination. Ice cores were paleoenvironmental and paleoclimatological records subsampled using a bandsaw frequently cleaned with etha- [20-27]. Characteristics present at UFG conducive to nol in cold room laboratories at NICL. Multiple core sec- preserving paleonvironmental signals include: (1) ice- tions from each interval were composited in order to coring site altitudes that exceed 4 km above sea level obtain sufficient S and N mass for isotopic analyses. The (ASL) to minimize meltwater modification of the snow surface ice from each subsample was scraped away with a and ice chemistry and (2) large ice thicknesses (ranging stainless steel microtome. Each ice sample was thoroughly from 60 to 172 m in the upper half of the glacier) to rinsed with ultrapure (18.0 megaohm) deionized water provide long-term paleoenvironmental records. and placed in a prerinsed and covered plastic container. Ice cores exceeding 160 m in length were recovered Each sample was allowed to melt at room temperature for from UFG in 1991 and 1998 [23,24]. The 1991 ice core one hour (or until approximately 15 mL of meltwater had was estimated to contain 250 years of record as deter- accumulated).